The abnormalities of male sex differentiation are relevant from chromosomal abnormalities, male differentiation with inappropriate karyotype, true hermophroditism and male pseudohermaphroditism (MPH). We define MPH as incomplete masculinization of the external genitalia in subjects with normal 46 XY karyotype. Clinical phenotype of MPH is not characteristic: it will depend on the degree of the abnormality and to its timing during fetal life. The syndromes resulting in MPH are: dysgenesis of the fetal gonads, abnormalities of gonadotropin, deficiency of enzymes needed for the biosynthesis of testosterone, and abnormality of androgen target cells. Some of these syndromes occur sporadically whereas others happen with a specific aggregation. Clinical data suggest that several syndromes of MPH are transmitted as an X-linked recessive trait, although the deficiencies of the enzymes necessary for the biosynthesis of testosterone have been reported to be transmitted as an autosomal recessive trait.